1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter fractions of a fluid sample. More particularly, this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation to cause separation of the heavier fraction from the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that overtime globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical reaction with the gel interface.
Therefore, a need exists for a separator device that (i) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) can be used with standard sampling equipment; (viii) is able to move into position to form a barrier in less time than conventional methods and devices; and (ix) is able to provide a clearer specimen with less cell contamination than conventional methods and devices.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly of the present invention comprises a plurality of constituents. Preferably, the assembly comprises a container, a liner and a composite element.
The container may be a conventional tube having a closed bottom, an opened top and a rigid cylindrical wall extending therebetween. The tube may include an inwardly directed rim near the open top.
The assembly further comprises a liner having a closed bottom, an open top and a tubular side wall. The liner is positioned in the tube such that the closed bottom of the liner is near the closed bottom of the tube. The liner, in an unbiased condition, is cross-sectionally dimensioned along most of its length to lie in spaced relationship to the tube. However, the liner may include an outwardly directed flange substantially adjacent the top of the liner. The flange may be dimensioned for engagement against the rim of the tube to position the liner longitudinally within the tube.
Preferably, the liner comprises a qualitative stiffness that may be characterized by a non-dimensional stiffness coefficient, S* and expressed as follows:       S    *    =            E      ⁡              (                  OD          -          D                )                    a      ⁢              xe2x80x83            ⁢              ρ        w            ⁢              D        2            
where E is the modulus of elasticity, OD is the thickness defined by the outside diameter, D is the seal diameter, a is the applied acceleration, and xcfx81W is the density of water. The stiffness coefficient is about 0.003 to about 190.
Preferably, the liner has a thickness of about 1.0 mm to about 2.5 mm, a modulus of elasticity of about 13.8 MPa to about 69 MPa.
Preferably, the assembly of the present invention will function under load created by an applied acceleration of about 300 g to about 3000 g.
Preferably, the liner deforms due to hydrostatic pressure under applied acceleration and returns to its initial state upon removal of the acceleration, thereby forming a seal by constricting the seal plug which is positioned in a target density region between the higher density portion and the lower density portion of a fluid sample.
The assembly further includes a tube closure that is sealingly engageable in the open top of the tube. The tube closure may include a tube end seat having an outside diameter at least equal to the outside diameter of the tube for disposition substantially adjacent the open top of the tube. The tube closure may include a tube stopper dimensioned for sealed engagement in portions of the tube between the top of the tube and the top of the liner. The tube closure may further include a liner stopper dimensioned for sealing engagement in the open top of the liner.
A plug recess extends into the bottom end of the tube closure. The entry to the plug recess may have a plurality of inwardly extending circumferentially spaced flexible walls.
Preferably, the composite element comprises a seal plug. The seal plug may be a single constituent or a plurality of constituents and comprises a specific density at a target density range as defined by separable fluid components densities. The seal plug may migrate freely when under an applied acceleration to settle at a location in the fluid sample in the target density region and thereby become a barrier at a desired level between the components of the fluid sample after the acceleration is removed.
Preferably, the seal plug has an aggregate specific gravity of about 1.028 to about 1.09. Most preferably, the seal plug has an aggregate specific gravity so that it will rest after centrifugal force, between the heavier and lighter phases of a blood sample.
The seal plug preferably has an overall density between the densities of two phases of a blood sample. The seal plug comprises a hard plastic shell having opposed first and second ends and an aperture extending between the ends. Outer circumferential portions of the hard plastic shell in proximity to the first end are dimensioned and configured for releasable engagement within the plug recess of the tube closure. Outer circumferential portions of the hard plastic shell in proximity to the second end are dimensioned for sealing engagement by the unbiased tube liner. The seal plug further includes an elastomeric septum that is securely mounted around the first end of the hard plastic shell to provide a pierceable barrier extending across the central passage through the shell.
Preferably, the seal plug comprises an overall specific gravity at a target specific gravity of "sgr"t. The target specific gravity is that required to separate a fluid sample into two phases.
In use, a fluid sample enters the assembly by a needle. The needle penetrates the closure and through the elastomeric septum on the seal plug for delivering a fluid sample into the liner. The needle is withdrawn from the assembly and the assembly is subjected to centrifugation. Forces exerted by the centrifuge cause the seal plug to separate from the tube closure and cause the liner to expand outwardly against the tube. Centrifugal forces then cause the seal plug to move through the expanded liner and toward the closed bottom of the tube. Sufficient movement will cause the seal plug to contact the fluid. Air trapped in the passage through the hard plastic liner and between the fluid and the elastomeric septum could create a buoyancy that might prevent further sinking of the seal plug into the fluid. However, the trapped air will be vented through a defect in the septum, such as the defect caused by the needle cannula. This venting of air permits further movement of the seal plug into the fluid. Simultaneously, the phases of the fluid will be separating such that the heavier phase component of the fluid will concentrate closer to the closed bottom, and such that the lighter phase component of the fluid will be closer to the open top. The seal plug will move primarily through the lighter phase component and toward the heavier phase component of the fluid.
The centrifuge may be stopped after the seal plug stabilizes between the separate phases of the fluid. Upon termination of the centrifugal load, the liner will resiliently return toward its unexpanded condition and will sealingly engage outer circumferential regions of the seal plug. As a result, the phases of the fluid sample are isolated from one another by the seal plug and may be separated for subsequent analysis.
When the fluid sample is blood, the higher specific gravity portion that contains the cellular components is between the separator and the bottom of the container after centrifugation. The lower specific gravity portion that contains the cell-free serum fraction or plasma is between the top surface of the separator and the top of the container after centrifugation.
Therefore, at the final position of the separator after centrifugation, the separator is able to substantially eliminate the presence of red blood cells in the lower specific gravity portion and the lower specific gravity is substantially free of cellular contamination.
The assembly of the present invention is advantageous over existing separation products that use gel. In particular, the assembly of the present invention will not interfere with analytes as compared to gels that may interfere with analytes. Another attribute of the present invention is that the assembly of the present invention will not interfere with therapeutic drug monitoring analytes.
Most notably, is that the time to separate a fluid sample into separate densities is achieved in substantially less time with the assembly of the present invention as compared to assemblies that use gel.
Another notable advantage of the present invention is that fluid specimens are not subjected to low density gel residuals that are at times available in products that use gel.
A further attribute of the present invention is that there is no interference with instrument probes.
Another attribute of the present invention is that samples for blood banking tests are more acceptable than when a gel separator is used.
Another attribute of the present invention is that only the substantially cell-free serum fraction of a blood sample is exposed to the top surface of the separator, thus providing practitioners with a clean sample.
Additionally, the assembly of the present invention does not require any additional steps or treatment by a medical practitioner whereby a blood or fluid sample is drawn in the standard fashion, using standard sampling equipment.